In an industrial plant context, there is a risk of formation of a relatively broad spectrum of waste. The term waste is used as a fairly broad term to refer to an accumulation of organic waste. One such organic waste includes sewage or sludge from a municipal sewage treatment facility. Accordingly, it will include high bacteria counts. Another material in the term waste as used herein is a relatively broad spectrum of various hydrocarbons. The hydrocarbons typically will include in random fashion various halogens, sulfur, the various light metal elements such as sodium and the like. It may also include the heavier metal ions typically deemed to be capable of metallic poisoning including compounds of lead, mercury and the like. Disposal of this type of waste covers a spectrum of needs including the necessity of killing the bacteria, breaking down the hydrocarbons, recovery of various salts and the like. In this context, the term waste therefore includes the typical waste products enumerated above. Other waste may well be added, and the mix or percentage of the various waste constituents can vary quite widely.
One proposed system for handling waste of this nature (perhaps more narrowly defined) is SAE Technical Paper 820872 bearing the title "Supercritical Water Oxidation for Wastewater Treatment: Preliminary Study of Urea Destruction" which bears the date of July 19-21, 1982. This paper describes an apparatus for use with extended duration space missions. It shows apparatus in FIGS. 1 and 2 thereof. It is the sort of device of rather narrow value as would be evident from its use in spacecraft. Another technical article is entitled "Supercritical Fluids" from pages 548A-551A, Volume 16, No. 10, Environmental Science & Technology published by the ACS. This technical paper involves the same personnel as were involved in the SAE paper mentioned above, and sets forth a device which can handle specified waste.
Patents of note include Bauer U.S. Pat. No. 3,449,247. This shows a wet oxidation procedure. A more recent patent is that of Lawless which bears U.S. Pat. No. 3,606,999. In this disclosure, a system for utilizing elevated pressures and continuous flowing streams (large volume) is set forth. The more recent patent of Titmas U.S. Pat. No. 3,853,759 is also noted.
The present disclosure sets forth both a method and apparatus for handling the large spectrum of waste described above. This disclosure sets forth an advantageous method and procedure utilizing an abandoned cased well. The term well is defined hereinafter as a well which is cased sufficiently to avoid damage to adjacent formations by bleeding the waste into the formations. Typically, this refers to a well which has been cased with the casing bonded to the adjacent formations by means of cement placed between the casing and the formation. To a large degree, the cement is also used as an insulator. It is preferably an insulator for the well to define a reaction chamber at the bottom of the well. The well is sufficiently deep that pressure at the bottom obtains a pressure above about 3200 psi. The water borne waste is heated sufficiently above 380.degree. C. and pressurized to an excess of about 3200 psi to be in the range where it is supercritical. In this range, gases dissolve quite readily into the water, and normally insoluble organics including greases and oils become soluble.
Such pressures can be achieved in relatively expensive reactor vessels. For instance, one such reactor vessel is offered by Modar, Inc., the firm described in the first of the two references mentioned above. Another type of vessel is an autoclave which is used by the firm known as Zimbro, Inc. of Rothschild, Wis. The various reactor vessels are relatively expensive to fabricate. They must of necessity sustain at least an internal pressure of 3200 psi. Providing over design, perhaps they can be designed for 3500 psi. Using that target pressure, the vessels are typically designed for even higher pressures to provide a safety margin and hence, they typically will have pressure ratings of 5000 psi or more.
Such a high pressure rating is inevitably expensive when translated into equipment. This typically requires the design and fabrication of relatively rugged pressure vessels. Such a pressure vessel is inevitably also small. For instance, the Modar equipment typically contemplates about 50 gallons per day throughput. While the model might be scaled up, there is a point of diminishing returns for cost. For instance, the capacity of a pressure vessel can be doubled with only a modest increase in the radius of the pressure vessel, that vessel being assumed to be a cylindrical structure. Even so, this leads to a ceiling or limitation in terms of the practical size of the plant. Plant size inevitably relates to throughput capacity. Moreover, such a plant typically must be continually fed with heat to sustain the temperatures which are necessary for operation. Inevitably, provision of the heat is expensive.
The present method and apparatus handles this matter entirely differently. This invention proposes the use of abandoned oil wells. They typically are drilled sufficiently deep to enable supercritical pressures to be obtained in the bottom area. In the event they are not that deep, they can nevertheless be used because the feed can be pressurized. Accordingly, the pressure at the bottom can be raised by incrementing the standing column of water with a pressure boost at the surface. Utilizing a rough rule of thumb that the pressure is increased by about one psi for every two feet of column height, a well which is approximately 6400 feet deep will furnish a bottom hole pressure of approximately 3200 psi. This can be done without pressurizing the well at the top. In this light, it should be recognized that the well encloses a standing column of water which increases the pressure to supercritical in the bottom or reaction area. The standing column is thus selectively boosted by providing a pressure head thereabove. While this pressure head does involve the installation of pressure retaining tanks, valves and the like connected at the well head, they are typically not so expensive as is the equipment necessary to contain 3200 psi at the surface. Rather, the surface equipment might provide a pressure boost of perhaps 500 psi. This would be helpful in a well that might be only 5400 feet deep.
As will be understood, the term pressure vessel is somewhat relative in this context. It is intended to refer to the bottom portions of an abandoned well. Preferably, the abandoned well is cased to prevent migration into the adjacent formations. Moreover, the casing is cemented in place to assure that the chamber at the bottom of the abandoned well will be available for continuous duty, use and operation. This is particularly important to enable industrial waste which typically is provided in an unending flow to be treated by cycling the waste through the apparatus of this disclosure.
This apparatus should be particularly considered from the point of view of start-up procedures. As will be described, the procedure is exothermic. However, assume for purposes of description that it is to begin with a column of water at ambient temperature placed in a wellbore. Assume for descriptive purposes that the wellbore is 8,000 feet deep. Assume further that it is enclosed in a casing and the casing is cemented in place. Assume further that the bottom of the well is plugged at 8,000 feet. This apparatus particularly sets forth a procedure for initiating operation. A centrally spaced bare conductor is located in the reaction chamber. The chamber is at 7,500 feet to 8,000 feet. This bare conductor injects a current flow which flows radially outwardly in the chamber. The conductor incorporates an insulated sheath extending from the surface down to the reaction chamber. In the reaction chamber, the bare conductor is centrally located and is held by standoff posts to assure that it does not contact the surrounding tubing. Moreover, it has an exposed length enabling radial current flow along the chamber. For instance, it can be centrally spaced by standoff posts made of insulated material not susceptible to breakdown, and the conductor and chamber might be fairly long, say 100 meters in length. If the tubing is approximately 75/8 id, preferably the conductor is sized and centered more or less along the center of this chamber. This enables the conductor to flow a current radially outwardly through the highly conductive material which surrounds the electrode.
The method of the present disclosure particularly sets out a procedure whereby the surrounding fluid is exposed to heating from current flow. As heat is liberated, the surrounding fluid is heated. As the temperature rises, flow of electric current is increased thereby further raising the temperature. In fact, current flow increases as a result of decrease in electrical resistance. Eventually, the temperature is raised by the electrical resistance heater arrangement to supercritical. Heat that is liberated from the exothermic oxidation thereafter sustains the temperature level. This oxidation of course goes further in disassociation of the complexes which form the waste. Bacteriological waste are completely sterilized by death and destruction of the bacteria.
The present invention takes advantage of the relative degree of insulation which is found in a cased and cemented well borehole. In deeper wells, the bottom hole temperature inevitably increases typically at a rate of about one degree F. per 100 feet depth. This increase provides a minor boost to the temperature of the fluid pumped to the bottom. More importantly, the heat transfer characteristics of the surrounding formation in conjunction with the cased and cemented borehole enables retention of heat to assure that the water in the reaction chamber is sustained at supercritical conditions. To this end, the water is mixed with injected air to thereby define a system wherein gaseous fluids are dissolved substantially with total solubility in the water, or restated, the water borne wastes and gases are perfectly soluble. The surface tension is reduced at supercritical to enable nearly perfect mixing, and fully contacts oxygen with the water borne waste materials. Oxygen can penetrate the smallest pores in particulate matter and oxidize all available material. This accelerates the combustion of the waste.
The by-products of waste consumption include carbon dioxide, additional water, formation of various salts including both soluble and insoluble salts. Basically, all cell life is destroyed. The discharge is therefore more readily handled. Toxic poisons, dangerous toxins and the like are avoided and the discharge is inevitably handled much easier. The discharge may include heavy metal salts which might have to be removed to provide a relatively clean effluent. This poses no problem in contrast with the problem that exists before treatment.
The foregoing sets forth both the method and apparatus of the present disclosure in very general terms. The procedure of use is also generally described. It will be appreciated that a better understanding of the present invention can be obtained on review of the drawings found below and the written disclosure.